Anti-lock Braking System - description

1 From 1986 onward an anti-lock braking
system is available as standard or optional
equipment on certain Escort models.

2 The system comprises four main
components: two modulators, one for each
brake circuit, and two rear axle load
apportioning valves, again, one for each brake
circuit. Apart from the additional hydraulic
piping the remainder of the braking system is
the same as for conventional models (see
illustration).

3 The modulators are located in the engine
compartment with one mounted on each side of
the transmission, directly above the driveshaft
inner constant velocity joints. Each modulator
contains a shaft which actuates a flywheel by
means of a ball and ramp clutch. A rubber
toothed belt is used to drive the modulator shaft
from the driveshaft inner constant velocity joint.

4 During driving and under normal braking,
the modulator shaft and the flywheel rotate
together and at the same speed through the
engagement of a ball and ramp clutch. In this
condition, hydraulic pressure from the master
cylinder passes to the modulators and then to
each brake in the conventional way. In the
event of a front wheel locking the modulator
shaft rotation will be less than that of the
flywheel and the flywheel will overrun the ball
and ramp clutch. This causes the flywheel to
slide on the modulator shaft, move inward and
operate a lever which in turn opens a dump
valve. Hydraulic pressure to the locked brake
is released via a de-boost piston allowing the
wheel to once again revolve. Fluid passed
through the dump valve is returned to the
master cylinder reservoir via the modulator
return pipes. At the same time hydraulic
pressure from the master cylinder causes a
pump piston to contact an eccentric cam on
the modulator shaft. The flywheel is then
decelerated at a controlled rate by the
flywheel friction clutch. When the speed of the
modulator shaft and flywheel are once again
equal the dump valve closes and the cycle
repeats. This complete operation takes place
many times a second until the vehicle stops or
the brakes are released.

5 The load apportioning valves are mounted
on the rear crossmember and connected to
each rear suspension arm via a linkage. The
valves regulate hydraulic pressure to the rear
brakes in accordance with vehicle load and
attitude in such a way that braking force at the
front brakes will always be greater than that at
the rear.

6 A belt break warning switch is fitted to the
cover which surrounds each modulator
drivebelt. The switch contains an arm which is
in contact with the drivebelt at all times. If the
belt should break, or if the adjustment of the
belt is too slack, the arm will move out closing
the switch contacts and informing the driver
via an instrument panel warning light.

See also:

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